Ultra Wide Angle Lenses for Sony Full Frame

Hey everyone!

I joined the forum because I was looking all over for a thread about wide-angle lenses for full frame cameras, but I couldn't find one! (correct me if I'm wrong though, and feel free to let me know: "just check some other thread".

So, anyways, I just got myself a Seafrogs housing for my Sony A7V since I will be going to the Maldives for an 8 day LOB (hopefully with beautiful pelagics!). I was looking at ultra wide angle options for this trip, but I couldn't make a decision and wanted to listen to some of your opinions.

As you read above, I'm on a Seafrogs housing so all the expensive Nauticam stuff won't work for me. I was initially thinking of using my Canon 8-15mm with a 2x converter (as recommended in some of the photography threads and it totally makes sense) and a Tokina converter (since the Sigma one doesn't do AF in video), but then I realized only after "putting all the converters into the cart", especially coming from a photography background, that the fisheye would be really weird / distracting on video! Although, I'm not sure if that's the case with Mantas, Whale Sharks and Nurse Sharks - and the reason I didn't refer to the reef sharks is because I think I will be using my 16-35 for the "reef-hook-dives" since I think some reach will be required there (and here, I'm really open to opinions since I've not been to the Maldives and the last time I went on a reef hook dive was in Tenggol Island in Malaysia looking for whale sharks with the 8-15mm, but it didn't really work since the whale shark was quite far).

That being said, I went on to look for some ultra wide rectilinear lenses, and read some nice reviews from @Phil Rudin (nice articles there!)

So, if I may ask - to shoot large stuff, like mantas, whale sharks, groups of nurse sharks would you recommend:
- Canon 8-15mm fisheye with / without 2x and which lens converter would you recommend (I was going for the Tokina because it was cheaper than Metabones, but not sure about AF speed difference) and not sure how the fisheye effect will play with large stuff.
- Perhaps the best way to shoot video is with rectilinear lenses and if so, I should consider ultra wide angle lenses. And if so, what do you recommend? I was looking at the Laowa 10mm / Laowa 12mm / Sony 12-24 / are there any other lenses to consider?

Thanks all in advance for taking the time!